An interesting program that I came across quite recently.

AirSnare monitors network traffic for unfriendly MAC addresses and alerts you when a MAC address is found that isn't on the friendly list. AirSnare also monitor DHCP requests from clients........ AirSnare is another tool to add to your Wireless Intrusion Detection Toolbox. AirSnare will alert you to unfriendly MAC addresses on your network and will also alert you to DHCP requests taking place. If AirSnare detects an unfriendly MAC address you have the option of tracking the MAC address's access to IP addresses and ports or by launching Ethereal upon a detection.
It is free, but the author calls it "beggarware"

To install this program, there are some things you will need:

1. A wireless network card;
2. The wireless client SSID set to the same SSID as the Access Point;
3. And a WinPcap Library on your machine.

Here's the link for further info on this tool


AirSnare - Intrusion Detection Software for Windows
Enjoy